5.5
Types de granulométrie
La granulométrie des granulats est essentielle pour obtenir de manière économique un mélange de béton présentant une résistance adéquate, une ouvrabil…
Le classement des agrégats signifie la distribution granulométrique des particules au sein d’un échantillon d’agrégats.
Les agrégats bien classés contiennent toutes les fractions granulométriques requises dans les proportions nécessaires pour former une matrice dense avec un minimum de vides.
La courbe de gradation correspondante est une courbe lisse continue, indiquant une large gamme de tailles de particules.
Lorsque la majorité des agrégats passent par un tamis spécifique et sont retenus sur le tamis plus fin suivant, cela signifie que les particules sont principalement de la même taille.
Il s’agit d’agrégats gradués uniformes ou de taille unique, représentés par une ligne presque verticale sur la courbe de gradation.
Les agrégats sont classés par écart si à peu près les mêmes fractions d’agrégats passent deux tamis différents de tailles considérablement différentes, indiquant l’absence de granulats dans la gamme de taille intermédiaire et est représenté par une ligne horizontale sur la courbe de gradation.
En l’absence d’agrégats fins, le mélange est dit à grade ouvert, ce qui entraîne de grands vides entre les particules grossières. La courbe de gradation correspondante est plate et presque nulle dans la gamme des petites tailles.
View the full transcript and gain access to JoVE Core videos
Q1: What does aggregate grading measure in concrete construction?
Aggregate grading signifies the particle size distribution within a sample of aggregate. It determines how different sized particles are distributed throughout the material. Understanding grading is crucial for economically obtaining a concrete mix with adequate strength, reasonable workability, and minimal segregation.
Q2: How do well-graded aggregates differ from uniformly graded aggregates?
Well-graded aggregates contain all required size fractions in proper proportions, forming a dense matrix with minimum voids and a smooth, continuous gradation curve. Uniformly graded aggregates consist predominantly of particles of the same size, depicted by a nearly vertical gradation curve. While well-graded aggregates ensure good workability, uniformly graded aggregates provide good permeability but lack stability.
Q3: What characterizes gap-graded aggregates on a gradation curve?
Gap-graded aggregates are characterized by the absence of certain intermediate size fractions. Approximately the same fractions pass two different sieves that differ considerably in size, resulting in a gradation curve with a nearly horizontal section. This missing material can lead to issues with concrete stability.
Q4: Why are open-graded aggregates highly permeable?
Open-graded aggregates lack fine particles that would normally fill voids between coarse particles. When fine aggregates are absent, the mixture results in large voids between the coarse particles, creating high permeability. However, this also reduces stability compared to other grading types.
Q5: How is a gradation curve used to identify aggregate grading types?
Gradation curves visually represent particle size distribution. Well-graded aggregates show a smooth, continuous curve; uniformly graded aggregates display a nearly vertical line; gap-graded aggregates exhibit a horizontal section indicating missing sizes; and open-graded aggregates show a flat curve with minimal values in smaller size ranges. These distinct curve shapes help classify aggregate types.
Q6: What is the relationship between aggregate grading and concrete workability?
Well-graded aggregates ensure good workability and stability by creating a dense matrix with minimum voids through proper size fraction distribution. In contrast, uniformly graded and gap-graded aggregates lack the complete range of sizes needed for optimal workability. Open-graded aggregates, while permeable, also compromise workability due to excessive voids.
Q7: Why does the absence of intermediate particle sizes affect concrete quality?
Gap-graded aggregates lack intermediate size fractions, creating discontinuity in the particle matrix. This absence prevents efficient packing and can compromise concrete stability and strength. Similarly, open-graded aggregates missing fine particles create large voids that reduce overall concrete quality and performance.